This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Japan Patent Application No.11-268664, No.11-269042 filed in Japan on Sep. 22, 1999 and No.2000-26056 filed in Japan on Feb. 3, 2000, the entire content of which is herein incorporated by reference.
1. Field of the Invention
This invention relates to a method of producing a tool steel, which is intended for use in manufacturing tools such as hot forging dies, extrusion dies and die casting dies, and a method of manufacturing tools from the tool steel, and the tool steel itself.
2. Description of the Related Art
Train wheels, automobile crankshafts and the like are generally manufactured by a hot forging which comprises heating a mass of steel at about 1,300xc2x0 C. and forging the steel into the product shape using dies. The technology of hot working includes, besides the above hot forging, a hot extrusion, by which steel bars and steel tubes are manufactured using dies. Among the dies used in hot working, there are dies used in casting aluminum alloys by the die casting method.
The tools such as the dies used in hot working processes undergo mechanical and thermal shocks at high temperatures. As a result, in addition to wear resulting from the friction between a die and a work in hot working, various cracks are formed on the tool such as the cracks, so-called heat checks, caused by a repetition of rapid heating and rapid cooling, the cracks caused by mechanical shocks, and the breaks resulting from the propagation of these cracks.
Therefore, a tool steel for hot working is required to have sufficient high temperature strength and fracture toughness rendering it resistant to wears, heat checks and breaks. The steel is also required to have good machinability so that the working time in tool manufactureing can be reduced and the life of the cutting tool to be used in manufacturing tools can be prolonged.
The tool steels in conventional use include alloy tool steels such as SKD61 and SKD62 based on the 5Crxe2x80x94Moxe2x80x94V steel, SKD7 based on the 3Crxe2x80x943Moxe2x80x94V steel, and SKT3 and SKT4 based on the Nixe2x80x94Crxe2x80x94Moxe2x80x94V steel, as defined in JIS G 4404. Under severe service conditions, however, these tool steels cannot meet such performance characteristics as mentioned above.
As a tool steel which can be used under such severe conditions, the applicant previously proposed a tool steel in JP Kokai H06-256897. The steel is characterized in that it contains, in percent by weight, C: 0.25 to 0.45%, Si: not more than 0.50%, Mn: 0.20 to 1.0%, P: not more than 0.015%, S: not more than 0.005%, Ni: 0.5 to 2.0%, Cr: 2.8 to 4.2%, Mo: 1.0 to 2.0% and V: 0.1 to 1%.
The chemical composition of this steel has been selected in order to obtain a martensite structure which is excellent in toughness and suitable for use in the form of dies. For its use as a tool, a method of obtaining dies has been disclosed which comprises the steps of oil quenching, tempering and working a steel into tool shapes.
The dies manufactured from the above tool steel have performance characteristics substantially satisfactory for use in hot forging dies and are quite applicable under ordinary hot forging conditions.
On the other hand, tool steels improved in machinability are disclosed in JP Kokai H04-358040 and JP Kokai H09-217147. The tool steel disclosed in JP Kokai H04-358040 is based on a technology of reducing the content of carbides which reduces machinability of the steel. However, a reduction of the carbide content results in reducing high temperature strength and therefore this tool steel has a drawback, for example the tool life is shortened.
The tool steel disclosed in JP Kokai H09-217147 reflects a technology of incorporating S and Te, which are alloy elements for enhancing machinability, into the steel as nonmetallic inclusions. In this technology, S and Te serve as a source of stress concentration in cutting work and thereby reduce the cutting force and increase the fracture facility of cutting tips, and thus attain an improvement of machinability. However, this tool steel has a disadvantage in that the nonmetallic inclusions of S and Te lead to a decrease in toughness and high temperature strength, although a certain extent of improvement in machinability can be noted.
It is an object of the present invention to provide a method of producing a tool steel, which is superior in high temperature strength and fracture toughness and in machinability to the conventional tool steels, and which can provide a prolonged tool life. A further object of this invention is to provide a method of manufacturing a tool from the tool steel and the tool steel itself.
The method of producing a tool steel according to the present invention comprises; preparing a steel having a chemical composition such that it contains, by mass percent, C: 0.25 to b %, Si: 0.10 to 1.20%, Mn: 0.20 to 1.50%, Ni: 0.50 to 2.00%, Cr: 1.00 to 4.20%, Mo: 0.30 to 2.00%, V: 0.10 to 1.00% and Al: 0.005 to 0.10%, with the balance being Fe and impurities, and further the content of P among the impurities is not more than 0.015%, that of S is not more than 0.005% and that of N is not more than 0.015%; quenching the steel to obtain a hardness H such that the hardness index K becomes between 0.20 to 0.95; and then tempering the steel.
The steel preferably has a chemical composition such that it contains, by mass percent, C: 0.25 to 0.45%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, Ni: 0.50 to 2.00%, Cr: 2.80 to 4.20%, Mo: 1.00 to 2.00%, V: 0.10 to 1.00% and Al: 0.005 to 0.10%, with the balance being Fe and impurities, among which the content of P is not more than 0.015%, that of S is not more than 0.005% and that of N is not more than 0.015%.
The steel also preferably has a chemical composition such that it contains, by mass percent, C: 0.40 to 0.60%, Si: more than 0.20% but not more than 1.20%, Mn: 0.20 to 1.50%, Ni: 1.00 to 2.00%, Cr: 1.00 to 2.70%, Mo: 0.30 to 2.00%, V: more than 0.10% but less than 0.80% and Al: not less than 0.005% but less than 0.10%, with the balance being Fe and impurities, among which the content of P is not more than 0.015%, that of S is not more than 0.005% and that of N is not more than 0.015%.
The method of manufacturing a tool according to the present invention comprises; preparing a steel having a chemical composition such that it contains, by mass percent, C: 0.25 to 0.60%, Si: 0.10 to 1.20%, Mn: 0.20 to 1.50%, Ni: 0.50 to 2.00%, Cr: 1.00 to 4.20%, Mo: 0.30 to 2.00%, V: 0.10 to 1.00% and Al: 0.005 to 0.10%, with the balance being Fe and impurities, and that the content of P among the impurities is not more than 0.015%, that of S not more than 0.005% and that of N not more than 0.015%; forming the steel into a tool shape; quenching the steel to obtain a hardness H such that the hardness index K becomes between 0.20 to 0.95; and then tempering the steel. The forming the steel into a tool shape may be carried out after tempering.
The steel for manufacturing a tool through the above-mentioned method preferably has a chemical composition such that it contains, by mass percent, C: 0.25 to 0.45%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, Ni: 0.50 to 2.00%, Cr: 2.80 to 4.20%, Mo: 1.00 to 2.00%, V: 0.10 to 1.00% and Al: 0.005 to 0.10%, with the balance being Fe and impurities, among which the content of P is not more than 0.015%, that of S is not more than 0.005% and that of N is not more than 0.015%.
The steel for manufacturing a tool through the above-mentioned method also preferably has a chemical composition such that it contains, by mass percent, C: 0.40 to 0.60%, Si: more than 0.20% but not more than 1.20%, Mn: 0.20 to 1.50%, Ni: 1.00 to 2.00%, Cr: 1.00 to 2.70%, Mo: 0.30 to 2.00%, V: more than 0.10% but less than 0.80% and Al: not less than 0.005% but less than 0.10%, with the balance being Fe and impurities, among which the content of P is not more than 0.015%, that of S is not more than 0.005% and that of N is not more than 0.015%.
The tool steel according to the present invention has a chemical composition such that it contains, by mass percent, C: 0.40 to 0.60%, Si: more than 0.20 but not more than 1.20%, Mn: 0.20 to 1.50%, Ni: 1.00 to 2.00%, Cr: 1.00 to 2.70%, Mo: 0.30 to 2.00%, V: more than 0.10 but less than 0.80% and Al: not less than 0.005 but less than 0.10%, with the balance being Fe and impurities, and further the content of P among the impurities is not more than 0.015%, that of S is not more than 0.005% and that of N is not more than 0.015%; and has a hardness H such that the hardness index K is between 0.20 to 0.95.
The hardness index K referred to hereinabove is defined by the following equation (1):
K=(Hxe2x88x92H2)/(H1xe2x88x92H2)xe2x80x83xe2x80x83(1)
where
H1: Vickers hardness found on a standard sample with 10 mm thickness which is heated to a temperature of the Ac3 transformation point plus 50xc2x0 C., and quenched into water;
H2: Vickers hardness found on a standard sample with 10 mm thickness which is heated to a temperature of the Ac3 transformation point plus 50xc2x0 C., and cooled slowly to room temperature over 20 hours.
The term xe2x80x9cquenchxe2x80x9d as used herein includes all treatments of cooling from the austenite zone.